Metal nanostructures have attracted significant attention during the past decades due to their ability to support localized surface plasmon resonances that can confine electromagnetic fields down to a deep-subwavelength scale and provide significant local-field enhancement [1,2]. They have led to many advances in both fundamental nanophotonic research and development of a variety of applications ranging from bio- or chemical sensing [3–5] and plasmon-enhanced spectroscopy [6–8] to nanophotonic devices (e.g., nanolasers and high-speed optical modulators) [9,10]. Recently, nanoparticle-on-mirror (NPoM) nanocavities, which can be readily fabricated by depositing metal nanoparticles onto a metal mirror covered with a nanometer-thick dielectric layer, have been intensely studied because of their ability to squeeze electromagnetic fields down to the single-nanometer or even sub-nanometer scale [11–14]. Therefore, they provide an unmatched nanophotonic platform for the study of light-matter interaction on the atomic scale, opening a new realm of possibilities for a broad range of applications including single-molecule strong coupling [15], enhancement of forbidden transitions [16,17], and nanoscale optoelectronic devices [18].

Due to the strong electromagnetic coupling between the metal nanoparticle and the mirror, NPoM nanocavities can support a rich variety of plasmonic modes across a broad spectral range [19,20]. In general, it is highly desired to selectively excite/enhance and precisely tune these modes to achieve the best performance of NPoM nanocavities for particular applications. For example, tuning a spectral position of a resonance mode to target the emission wavelength of emitters integrated in the nanocavity (e.g., dyes, rare-earth ions, or quantum dots) can be used to maximize the Purcell effect [21,22], while enhanced excitation of the specific resonance mode can promote selective interaction with the desired electronic state of matter (e.g., excitons) [23,24]. Theoretically, the optical response of plasmonic nanocavities can be precisely controlled by adjusting the size and shape of the nanoparticles, the thickness of the dielectric layers and metal mirrors, as well as the excitation conditions (e.g., the incident angle and polarization of the incident light) [12,19,25,26]. However, due to extreme sensitivity of the strongly confined optical fields in NPoM nanocavities to their structural parameters, in practice, it is challenging to obtain NPoM nanocavities whose optical response is the same as the desired. Therefore, an approach for precise engineering of the optical response of NPoM nanocavities at the post-fabrication stage is highly desirable. Several methods have been developed towards this goal, including laser-induced reshaping of the metal nanoparticles in order to enrich the supported plasmonic modes [27] or integrating an active layer with an electrically tunable refractive index into the nanocavity gap to tune the mode resonance wavelengths [28,29]. However, the former approach usually results in an irreversible alteration to the metal nanoparticles and a potential damage to the functional materials frequently integrated into the gap, while the latter shows only limited tunability.

Here, we demonstrate precise control of the plasmonic response of nanocube-on-mirror (NCoM) plasmonic nanocavities by embedding them into a poly(methyl methacrylate) (PMMA) layer with a variable thickness. Single-crystal gold nanocubes (∼60nm in size) and flakes were used to construct the NCoM nanocavities with low loss and uniform spectral response. As the PMMA thickness increases from 0 to approximately 100 nm, the dominant out-of-plane plasmonic modes are markedly suppressed and eventually vanish from the scattering spectrum, while the in-plane plasmonic modes are significantly enhanced, with their intensity increasing by a factor of ∼100. This enhancement stems from the variation of momentum matching between the plasmonic modes and the radiative fields, affecting both mode excitation and emission properties, as confirmed by the theoretical simulation results. In addition, the resonant peak wavelengths of the in-plane and out-of-plane plasmonic modes are tuned up to ∼52 and 81 nm, respectively.

In order to obtain plasmonic nanocavities with low loss and uniform spectral response, chemically synthesized single-crystal gold nanocubes with high uniformity in size [30] and gold flakes with an ultrasmooth surface (having a typical surface root-mean-square roughness of ∼0.2nm [19,31]) are used as building blocks for the construction of the NCoM plasmonic nanocavities [18], as shown in Fig. 1(a) (see Appendix A for the fabrication details). Figure 1(b) presents a cross-sectional transmission electron microscopy (TEM) image of an individual NCoM nanocavity (see Appendix B), clearly showing that the gold nanocube (∼60nm in size) and the gold flake (∼50nm in thickness) are well separated by a dielectric layer, which consists of a 1.8-nm-thick bilayer of cetyltrimethylammonium bromide (CTAB) capping the nanocubes and a 2.5-nm-thick alumina spacer deposited on the flake. Under the illumination with a TE- and TM-polarized white light at the angle of incidence of 80° (see Appendix C for the experimental details) [32–34], the nanocavities exhibit uniform green [Fig. 1(c)] and doughnut-shaped red [Fig. 1(d)] scattering, respectively. Particularly, under the TE-polarized illumination, there are two resonance peaks located at the wavelengths of 527 and 725 nm in the scattering spectrum [labeled as M1 and M2, respectively; red line in Fig. 1(e)]. The observed modes are reproduced very well in the simulated scattering spectrum and correspond to the flake-coupled quadrupolar and transversal dipolar modes of the nanocube, respectively (see Appendix D for the simulated scattering spectrum and the corresponding charge density distributions) [19,20]. These two modes are poorly coupled to light and therefore can be regarded as dark. They are excited by the in-plane component of the electric field. Under the TM-polarized illumination, another two scattering peaks located at the wavelengths of 581 and 660 nm appear [labeled as V1 and V2, respectively; black line in Fig. 1(e)], which stem from hybridization of the flake-coupled vertical dipolar mode of the nanocube and one of the second-order metal-insulator-metal (MIM) modes in the nanocavity gap (see Appendix D for the corresponding charge density distributions) [19,20,35]. Different from modes M1 and M2, these two modes are bright and preferably excited by the out-of-plane component of the electric field. In addition, the slight difference in the peak wavelengths for mode M2 under TE and TM excitations arises from a slight difference in the in-plane edge lengths of the nanocube. Benefiting from the excellent structural quality of the plasmonic building blocks and the uniform thickness of the deposited alumina layer, the as-fabricated NCoM nanocavities show uniform plasmonic resonances in the scattering spectra [Fig. 1(f)].

The optical response of the NCoM plasmonic nanocavities can be readily engineered by embedding them into a dielectric layer with a controllable thickness. Experimentally, it is realized by first spin-coating a 96-nm-thick PMMA layer onto the NCoM nanocavities (to ensure the complete embedding), and then using O2 plasma etching to gradually tune its thickness (dp), as schematically shown in Fig. 2(a) (see Appendix A for the fabrication details). The thickness in this case can be precisely controlled with a precision of ±3nm by setting the treatment time and referring to the thickness-etching time correlation curve of the PMMA layer [as presented in Fig. 2(b); see Appendix E for the measurement details]. In the case of over-etching, the PMMA layer can be etched completely first, and a new PMMA layer (∼96nm) can be spin-coated onto the same nanocavities and then etched to readjust the PMMA thickness. Their relative positions and optical response remain unchanged after several rounds of spin-coating and etching of PMMA layer (see Appendix F for details). As a result, the evolution of the scattering spectrum of a PMMA-embedded NCoM nanocavity can be obtained by progressively decreasing the PMMA thickness dp, as shown in Figs. 2(c) and 2(d). In the case of TE-polarized light illumination when only modes M1 and M2 are excited, with the decrease of dp, the resonance wavelengths of the modes are gradually blue-shifted. This is accompanied by a decrease of their scattering intensities, which can be intuitively seen on the dark-field scattering images shown in the lower panels in Fig. 2(c). Under the excitation with TM-polarized light, modes M1 and M2 dominate the scattering spectrum for the thicknesses of the PMMA layer above ∼50nm, while with further decrease of the thickness, modes V1 and V2 appear and their scattering intensities drastically increase (especially that for mode V2). The corresponding dark-field scattering images [lower panels in Fig. 2(d)] demonstrate the trends observed in the spectra, with the red doughnut-shaped component (corresponding to mode V2) gradually appearing with the decrease of the PMMA thickness under the TM excitation, and then dominating the pattern for the thin (less than 36 nm) PMMA layers. With the decrease of dp, the resonance wavelengths of modes M1,M2, and V2 also gradually blue-shift. At the same time, mode V1, which is less radiative in nature for its anti-bonding characteristic, is only observable when the thickness of the PMMA layer is below 36 nm due to the spectral mismatch between the vertical dipolar mode of the nanocube and the second-order MIM mode in the gap for thicker PMMA layers [35].

To get an insight into the effect of the thickness of the PMMA layer on the optical response, finite element numerical simulations of light scattering on NCoM nanocavities embedded in a PMMA layer with a varied thickness were performed. Figures 3(a) and 3(b) present the calculated scattering spectra under TE-polarized and TM-polarized light illuminations, respectively, and consistently reproduce the experimental observations [Figs. 2(c) and 2(d)]. The spectral positions and scattering intensity enhancement of the plasmonic resonances extracted from the experimental and calculated scattering spectra for modes M1,M2, V1, and V2 are then quantitatively compared in Figs. 3(c)–3(e). The scattering peak intensity ratio is defined as Ip/I0, where Ip and I0 are the peak scattering intensities of the plasmonic modes for NCoM nanocavities with various PMMA thicknesses dp and that without the PMMA layer, respectively. As discussed above, for mode M1, with the increase of dp from 0 to 96 nm, its resonance wavelength experiences a gradual red-shift from 525 to 578 nm, while for mode M2 a visible red-shift from 727 to 768 nm only occurs when dp is less than 48 nm [Fig. 3(c), solid dots]. This is understandable because for mode M2 the electromagnetic energy is mostly confined in the nanocavity gap, while for mode M1 the electromagnetic energy is confined not only in the nanocavity gap but also around the top surface of the nanocube, which can be clearly seen on the corresponding near-field maps shown in Figs. 4(a), 4(b), 4(d), and 4(e). The calculated evolutions of the resonance wavelengths reproduce the experimental results very well [Fig. 3(c), dashed lines]. More interestingly, with the increase of dp from 36 to 84 nm, there is a significant enhancement of the peak scattering intensity for mode M1; the measured scattering peak intensity ratio reaches a maximum value of 102±20 when dp is 84 nm [Fig. 3(d)]. The slight difference between the experimental and calculated results may occur due to the additional surface roughness of the PMMA layer introduced by the O2 plasma etching used in the experiments. Mode M1 with electromagnetic energy confined also around the top surface of the nanocube is sensitive to the surface roughness of the PMMA layer, resulting in an enhancement in the scattering of near-field electromagnetic energy into the far field. The significant enhancement in the scattering intensity of mode M1 with an increase of the PMMA thickness can be attributed to the simultaneous rise of the excitation and out-coupling efficiencies. On one hand, the increase of the surrounding refractive index leads to the increase of the wave number of the incident light, which enlarges the field retardation effects, and therefore boosts the excitation efficiency of mode M1 (which, as noted above, originates from the coupling of a quadrupolar mode of the nanocube with the gold flake) [36,37], as shown in Figs. 4(a) and 4(b). On the figure one can observe that the retardation-induced effects lead to an ∼200% enhancement of the excited mode electric field (defined as a ratio between the local |E| and incident |E0| electric fields). On the other hand, the increase in the surrounding refractive index exploiting the same retardation effects also reduces the momentum mismatch between the mode highly confined in the nanocavity gap and the free-space light, which results in an increase of the mode radiation efficiency [38]. The latter phenomenon was confirmed by calculation of radiation efficiency for modes M1 and M2 using eigen-mode simulations, which were performed using built-in eigen-frequency [the radiation efficiency was calculated as Pr/(P+Pabs), where Pr, P, and Pabs represent far-field power flow inside a 64° collection angle corresponding to the NA of the objective, and total far-field power flow and absorbed power, respectively, for NCoMs with various PMMA thicknesses dp; see Appendix D for the simulation details]. The radiation efficiency for mode M1 increases from 0.8% to 23.1%, reaching a maximum enhancement value of ∼29 at dp=96nm [Fig. 4(c)], which confirms that the out-coupling efficiency is another important factor for the scattering intensity enhancement. Comparing the excitation and radiation efficiencies with the theoretical scattering enhancement of ∼56, one can conclude that for mode M1 the radiation enhancement plays the major role.

With the increase of dp from 0 to 96 nm, the peak scattering intensity of mode M2 shows a similar trend to that of mode M1, but with a significantly reduced enhancement factor, which reaches its maximum value of ∼11±4 in the experiment and ∼10 in the simulations at the PMMA thickness of 96 nm [Fig. 3(d)]. This shows that for mode M2, which is strongly localized in the nanocavity gap for all the PMMA thicknesses [Figs. 4(d) and 4(e)], the retardation has a negligible effect on the excitation efficiency. Comparing the overall scattering enhancement with the radiative counterpart [increasing from 9.9% to 21.2% as dp increases from 0 to 96 nm, Fig. 4(c)], one can conclude that for mode M2 the excitation and radiation enhancements have comparable contributions.

In dramatic contrast to the in-plane modes M1 and M2, the evolution in the peak scattering intensities of the out-of-plane modes V1 and V2 shows the opposite behavior. Particularly, mode V1 keeps its resonant wavelength around 598 nm with the increase of dp from 0 to 36 nm [orange lines in Fig. 3(c)], while its peak intensity decreases gradually [Fig. 3(e)]. Mode V2, which dominates the scattering spectrum of the nanocavity under the TM-polarized illumination in the absence of the PMMA layer, undergoes a progressive red-shift from 660 to 726 nm with the increase of dp from 0 to ∼48nm [blue lines in Fig. 3(c)] and then spectrally overlaps with mode M2. At the same time, its peak intensity dramatically decreases to a nearly zero value [Fig. 3(e)]. According to the theoretic calculation results, with the increase of dp from 0 to 36 nm, there is ∼4 and 1.7 times decrease in the mode local intensity [Figs. 4(f) and 4(g)] and radiation efficiency [Fig. 4(c)] for mode V1, which indicates that the decrease of the excitation and radiation efficiencies plays a comparable role in the overall effect. Comparing the corresponding quantities for mode V2 [Figs. 4(c), 4(h), and 4(i)], one can also conclude that in this case the reductions of excitation (∼2 times decrease) and radiation (∼1.5 times decrease from 18.7% to 12.7%) efficiencies have comparable contributions.

By embedding NCoM plasmonic nanocavities into a PMMA layer with a controllable thickness, we have demonstrated selective excitation of their plasmonic resonances, achieving a scattering intensity enhancement up to ∼102 times for the normally dark in-plane plasmonic modes. This can be exploited for enhancing interaction of light with in-plane oriented matter states at the atomic scale. The main advantage of this approach is that the deposition and etching of the PMMA layer have negligible effects on both the nanocavity structure and functional materials frequently integrated in the nanocavity gap, enabling robust engineering and enhancement of light-matter interactions at the atomic scale. More interestingly, the insulating PMMA layer with a precisely controllable thickness partially surrounding nanoparticles opens a prospective for electric connection with nanoparticles featuring a sub-100 nm footprint [18,39–41]. Moreover, if the embedding medium PMMA is replaced with a humidity-sensitive polymer, it can undergo refractive index and volume changes in response to ambient humidity [42,43]. This, in turn, alters the plasmonic resonances of the nanocavity, offering a promising route toward optical humidity sensing with high sensitivity and high spatial resolution. Overall, the demonstrated approach for plasmonic mode manipulation through highly controllable embedding of plasmonic nanostructures into dielectric nanolayers provides a universal platform for the enhancement of light-matter interactions and opens opportunities for improving the performance of nanophotonic and quantum devices.